The present invention relates generally to practical thin film resistor structures and methods for integrating multiple thin film resistors of the same or different sheet resistances and the same or different temperature coefficients of resistance, and to providing a practical means of adjusting the temperature coefficient of integrated circuit components.
Design engineers would be able to better optimize some integrated circuit designs if it were practical and economical to integrate thin film resistors having various sheet resistances and TCRs (temperature coefficients of resistance) into a single integrated circuit structure. However, there has been no practical, economical way to accomplish this because temperature processing cycles associated with forming subsequent thin film resistor layers subsequent to formation of a first thin film resistor layer would cause a variety of difficult integrated circuit processing problems. For example, controlling the effect of various thermal cycles on the sheet resistances and TCRs of the multiple thin film resistors formed on successive oxide layers may be very difficult. Also, the presence of metallization layers in integrated structures including thin film resistors on multiple layers may make it very difficult to design subsequent thermal cycles of the kind needed to be compatible with the thin film resistor properties. Design engineers frequently find it desirable to use a resistor having a large positive TCR to offset a negative TCR of a another circuit element. However, there has been no practical way of providing high-precision thin film resistors having positive TCRs in typical integrated circuit structures.
U.S. Pat. No. 4,019,168 entitled “Bilayer of Thin Film Resistor and Method for Manufacture”, issued Apr. 19, 1977 to Franklyn M. Collins, describes an integrated circuit structure including a layer of tantalum on a layer of nichrome for the purpose of stabilizing the sheet resistance of the nichrome. However, the foregoing patent is not directed to issues regarding the TCR of thin film resistors.
It is conventional to adjust the thickness or sheet resistance of a resistive thin film layer by using suitable thermal anneal cycles to achieve a target sheet resistance and a target TCR for a deposited SiCr layer. Empirical curves have been developed that represent the relationships between the TCR and sheet resistance of various resistive thin film materials as functions of various integrated circuit processing parameters, such as the type of resistive material, thermal cycle temperatures and durations, etc. Once the sheet resistance of a thin film layer is known, the amount of annealing needed to increase its TCR by a desired amount can be determined from the curves. However, the technique of using thermal cycles to obtain a TCR target value that is precisely equal to zero or other value is not practical for some materials and for some sheet resistances, especially for SiCr, and especially for making very low resistance thin film resistors which are very wide and very short and therefore are of substantially reduced accuracy.
There is an unmet need for a practical method and integrated circuit structure for providing various combinations of the same or different high or low sheet resistances and the n same or different TCRs for two or more thin film resistors, each on a different player.
There also is an unmet need for a practical technique for providing a thin film resistor structure that can be used to offset the TCR of integrated circuit elements in which the TCR is not easily adjustable during integrated circuit manufacture.
There also is an unmet need for a way of manufacturing more stable thin film resistors in an integrated circuit process.
There also is an unmet need for an integration technique for providing two accurate SiCr thin film resistors having different sheet resistances and zero-value TCRs.
There also is an unmet need for an integration technique for providing two accurate SiCr thin film resistors, at least one of which has a precisely determined TCR, in an integrated circuit structure.